1. Field of the Invention
The invention relates to a joining method and a joining apparatus for joining a resin and a metal by heating.
2. Description of the Related Art
Resin members and metal members are usually joined together by using an adhesive, but in order to simplify the joining process and comply with the Volatile Organic Compounds (VOC) regulations, physical concavities and convexities are formed on the joining surface of the metal member with the resin member or chemical functional groups are modified by performing surface treatment such as acid or alkali treatment or a primer treatment, or a joint body of a resin and a metal is obtained by conducting insert molding of a thermoplastic resin on the joining surface of the metal member with the resin member.
In a case where a metal member and a resin member are joined together by a process in which, as mentioned above, the joining surface of the metal member with the resin member is subjected to a surface treatment, although no adhesive is used, a large amount of surface treatment agent such as an acid, an alkali, or a primer treatment agent is used and, therefore, when the spent surface treatment agent is discarded, environmental load is created. In addition, the surface-treated metal member has to be cleaned and dried, and a long time is required for the cleaning and drying operations. As a result, the aforementioned process is unsuitable for parts that require high productivity, such as automotive parts or the like. A problem associated with a process in which a metal member and a resin member are joined by insert molding a thermoplastic resin on the metal member, is that limitations are placed on the shape of the joint body.
Accordingly, for example, Japanese Patent Application Publication No. 5-185521 (JP-A-5-185521) describes a method for joining a resin member and a metal member within a short period by a simple process without using an adhesive or a surface treatment agent and without creating environmental load, wherein the resin member is joined to the metal member by heating and melting.
In a case where the resin member is heated, as mentioned hereinabove, to a melting temperature or softening temperature, the softened resin member deforms according to concavities and convexities of the metal member surface and the two members are joined by the anchor effect demonstrated due to such deformation, but a sufficient joining strength cannot be obtained in joining by the anchor effect alone. Furthermore, the resin member is heated by a hot atmosphere inside an oven or with a heater, but if the heating temperature is too high, gas bubbles are generated inside the resin and these bubbles cause crack generation in the resin member after joining. In addition, where the entire resin member is heated, the resin member is entirely melted and design properties of the outer surface of the resin member are lost. The resultant problem is that the application range of the joint of the resin member and metal member is limited.